Design and Engineering of Gibbs Free Energy for Heterostructure Nano-Bonding™ of Piezo-Electric Crystals to Si-based materials at T < 500 K

Forming heterostructures between piezo-electrics such as LiTaO₃ and LiNbO₃ with Si-based materials is limited by the mismatch of their lattice constants and thermal expansion.

This work uses RT Nano-Bonding™ [1] to bond piezo-electrics to Si via Surface Energy Engineering (SEE). SEE modifies synergistically Surface Energies (SE) and hydro-affinity to far-from-equilibrium states create reactive 2D Precursor Phases (2D PP) on each surface of the pair. 2D PPs react in air at RT when contacted by water vapor catalysis.

The △G_Piezo-Si-H2O for each pair is computed from SE measured via Three Liquid Contact Angles to see whether it favors Nano-Bonding™. The △G_Piezo-Si-H2O for ‘as received’ (AR) LiTaO₃, with ‘after SEE’ Si and α-quartz SiO₂ is -0.1 mJ/m² and -16.8 mJ/m². For or AR LiNbO₃, with ‘after SEE’ Si and α-q SiO₂ it is -13.4 mJ/m² and -28 mJ/m², thus all < 0.
Nano-Bonding™ of AR hydrophobic piezo-electrics with ‘after SEE’ hydrophilic Si/ SiO₂ is thus favored in the presence of atmospheric moisture.
Experimentally, 2D PP generated by SEE are indeed found to activate Nano-Bonding™, at RT in 25 % RH.

[1] Nano-Bonding™, Herbots, et al. U.S. Pat. # 9,018,077 (2015), 9,589,801(2017), pend. (2020)